This invention relates generally to adaptive optics wavefront control systems. More particularly, this invention relates to a new and improved adaptive optics wavefront control system wherein phase errors are reduced to a minimum using a coarse/fine gradient sensor.
Optical wavefront sensors known as Hartmann sensors are well known in the art. An example of a Hartmann wavefront sensor is described in U.S. Pat. No. 4,141,652. This type of sensor is composed of an array of wavefront gradient (tilt) sensors and a reconstructor. The tilt magnitudes in X and Y directions are measured in each of a number of subapertures which are contiguous with no appreciable gaps existing between subapertures. By a process of two dimensional numerical integration, the tilt measurements may be combined to reconstruct a wavefront phase map in which high spatial frequencies are missing because of the smoothing action of averaging tilt over the area of each subaperture.
A diagram of a Hartmann optical wavefront sensor is shown in FIG. 1 at 11. In FIG. 1, the input optical beam 10 is roughly collimated and falls upon a lenslet array 12. The lenslet array is a closely packed, two dimensional array of lenses 14. Each lens 14 focuses a portion of the input beam (called a subaperture) onto a two-dimensional array of position sensitive detectors 16. The detector array 16 can be formed by suitably mounting individual quadrant cell trackers, one for each lenslet (subaperture), or by a monolithic array of photosensitive pixels, such as are available in charge injection devices (CID) or charge coupled device (CCD) detector arrays.
The intensity of light falling on one subunit or pixel of the detector array 16 is read out in the form of an electronic charge or current into a centroid computer 18. After reading out the electronic signals (proportional to the light impinging on each pixel) corresponding to all the pixels in the array of detectors 16 into centroid computer 18, the centroid computer calculates through either analog or digital computing the first moment of the intensity distribution in both X and Y directions for each subaperture. This is the intensitY centroid and, if the lenslet arrays have reasonably good optical quality, is proportional to the input wavefront tilt averaged over the subaperture area of each lenslet 14.
A wavefront reconstructor 20 receives the X and Y centroid positions for each subaperture which, when multiplied by a suitable conversion factor, represents the subaperture wavefront tilts. Reconstructor 20 can be analog in operation, such as an array of resistors driven by current sources for each tilt measurement. In this case, the wavefront phases can be recovered at the array of points between the subapertures by measuring the voltages present at the nodes of the resistor array. Another implementation is a digital computer which performs the numerical integration of the tilts by matrix multiplication to produce an array of input phase estimates.
A disadvantage of the two-dimensional detector array 16 is the large number of pixels required to achieve a useful dynamic range of input wavefront deviations from the nominal shape, which is usually taken as planar. That is, at least three, and normally four pixels are required to measure the X and Y centroid coordinates for each subaperture (and therefore the subaperture wavefront tilt). If sufficient sensitivity could be achieved with this minimum number of pixels, then the detector could be read out quickly in series with a fast response time. This is necessary to achieve a large temporal bandwidth when the wavefront sensor is used as part of an adaptive optics wavefront control system.
In known adaptive optics control systems, the input beam is first reflected off an adjustable mirror (such as a deformable mirror with an array of actuators to introduce changes in the shape of the mirror reflecting surface) and then fed into the wavefront sensor 11. A closed-loop servo control system with negative feedback is used to control the reflection angle off each subaperture's part of the input optical beam so as to minimize the deviations of each centroid from its nominal position on the array detector 16. Such a closed-loop system for controlling the adaptive optics control loops is shown in FIG. 2.
In FIG. 2, the input optical beam 22 is reflected off deformable mirror 24 whose shape is controlled by piston actuators 26. Next, the light beam is passed through a high-quality beam splitter 28 with negligible optical aberrations. A first portion 30 of the light is reflected by beam splitter 28 to form the compensated output optical beam 30. A second portion 32 of the light reflected by deformable mirror 24 is transmitted by beam splitter 28 to wavefront sensor 11 (which is the device shown in FIG. 1). The reconstructed wavefront phase deviations from the desired (planar) shape serve as error signals to identical negative-feedback servo control loops, one for each phase measurement point and its corresponding piston actuator 26 in the deformable mirror assembly. The servo electronics 33 receive the wavefront phase error signals, process them by multiplication and normally by integration and frequency-dependent filtering to achieve high gain and freedom from undesirable oscillations. Servo electronics 33 also drive the actuators 26 in the direction to reduce the wavefront phase errors. It will be appreciated that under steady state conditions, the surface of deformable mirror 24 is driven to the conjugate of the input beam wavefront shape so that upon reflection, the light is equiphase across the beam both going onto the wavefront sensor 11 and also at the beam control system output 30.
As previously mentioned, the spatial dynamic range and sensitivity of the subaperture centroid detectors 16 may not be sufficient to meet two fundamental requirements for the beam control system of FIG. 2 to operate properly. First, the centroid trackers must have sufficiently large dynamic range such that, when the system is first activated, an unambigous measurement of each centroid position of each lenslet spot 14 is obtained. If the input beam has large subaperture tilts, the spots may overlap or appear so far from their nominal positions that either it is impossible to tell which spot belongs to which subaperture (in the case of a CID/CCD array); or the spot misses the detector altogether and no centroid determination is possible. On the other hand, if a very large tilt dynamic range is achieved by, for example, using a lenslet 14 with a very short focal length, then noise sources such as shot noise, dark current, nonlinearity, charge transfer inefficiency, quantization, etc. will limit the precision with which the centroid can be determined even near the null operating point. In this case, the closed-loop operation may be limited by the lack of sensitivity, with the result being that the output beam has wavefront deviations due to noise sources internal to the wavefront sensor 11 which are clearly undesirable.
For a closed-loop beam control system such as is shown in FIG. 2, the large capture range and the high sensitivity required in the subaperture tilt sensor can be achieved in several ways. One method is to use a larger number of pixels, arranged so that the spot diameter is larger than one pixel. This allows centroid determination to a small fraction of a pixel size. Using many such pixels (for example, an 8.times.8 array) will allow a large dynamic capture range for each spot without confusion. However, this method inevitably results in a reduced temporal bandwidth since many more pixels must now be read out which takes a correspondingly longer time period. Alternatively, many fewer subapertures could be sensed in the same time, but again this is highly undesirable since higher spatial frequency information about the wavefront shape will be lost. Thus, one must give up either high temporal or high spatial frequency information with this method if the readout rate of array detectors is assumed to be constant.